As its overall goal this project seeks to understand the pathogenesis of nontoxic goiter, a common clinical problem which may lead to thyroid nodules and thyroid cancer. We are focussing particularly on thyroglobulin, the major protein of the thyroid, which serves both as matrix for hormone synthesis and as storage depot once hormones are formed. We are particularly interested in whether variations in thyroglobulin structure can lead to impaired thyroxine synthesis and goiter, and if so, what mechanisms are involved. The relation between thyroglobulin structure and thyroxine synthesis is being approached in several ways. (1) We are trying to define the chemical requirements for normal thyroxine biosynthesis in thyroglobulin. We have shown that the amino acid environment of thyroxine in thyroglobulin is restricted rather than random. We have fractionated thyroglobulin into subunit pools, which we are purifying further, to assess their role in thyroxine synthesis. (2) As part of the study of thyroglobulin structure, we are examining the products and mechanisms of thyroglobulin proteolysis. (3) This project has demonstrated that TSH alters the amino acid composition of thyroglobulin, as well as the distribution of its iodoamino acids and its subunit structure. (4) Thyroglobulins from normal and goitrous glands are being isolated and studied individually. We have found that there are significant variations among the amino acid compositions of "normal" thyroglobulin, indicating structural heterogeneity for this protein. Thyroglobulins from goitrous glands and from thyroid carcinoma have shown more extensive deviations from the mean of the "normals." The relationship of these changes to thyroid disease is being examined further.